Phosphoric acid purification

ABSTRACT

WET ACID IS FREED OS SUBSTANTIAL MAGNESIUM IMPURITIES BY CONTACTING THE ACID WITH A SOLUBLE HEXAFLUOROSILICATE (SIF6-1) THEREBY FORMING A PRECIPITATE OF HYDRATED MAGNESIUM HEXAFLUOROSILICATE (MGSIF6.6H20) WHICH IS THEN REMOVED, LEAVING THE PURIFIED ACID.

United States Patent 01 fice 3,819,810 Patented June 25, 19 74 3,819,810 PHOSPHORIC ACID PURIFICATION David Goldstein, East Brunswick, N.J., assiguor to FMC Corporation, New York, N.Y.

Continuation-impart of abandoned application Ser. No.

108,920, Jan. 22, 1971. This application June 19, 1972,

Ser. No. 264,177

Int. Cl. C01b 25/16 US. Cl. 423321 4 Claims ABSTRACT OF THE DISCLOSURE Wet acid is freed of substantial magnesium impurities by contacting the acid with a soluble hexafluorosilicate (SH- thereby forming a precipitate of hydrated magnesium hexafluorosilicate (MgSiF -6H O) which is then removed, leaving the purified acid.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of Ser. No. 108,920, filed Jan. 22, 1971, now abandoned.

This invention relates to the purification of phosphoric acid produced by the wet process. The invention is particularly concerned with the removal of metal contaminants from such acid.

The manufacture of phosphoric acid by the so-called wet process has been practiced for many years in the chemical industry. In this process, crushed phosphate rock is acidified with sulfuric acid thereby releasing the phosphoric acid. The reaction is generally regarded as proceeding according to the following equation assuming the phosphate rock is fluoroapatite:

where n is 0.5 to 2.0. The calcium sulfate is separated, usually by filtration, and the aqueous acid which assays at about 30% P and known in the trade as filter acid, concentrated to the desired strength. Merchant acid, having 3. P 0 content of 52-54% is normally produced. Another important product made from wet acid is superphosphoric acid where further evaporation is carried out to a total P 0 content of 70-72% to give a product containing substantial amounts of pyro and other condensed polyphosphoric acids.

In actual practice, however, the fluoroapatite phosphate ore contains other mineral substances such as certain metal oxide assays e.g. MgO, CaO, Fe O and A1 0 and these find their way into the crude wet acid product thereby rendering it unsuitable for many applications. For instance, the presence of MgO in excess of about 0.5% in a superphosphoric acid containing 70-72% P 0 is especially objectionable due to the precipitation of magnesium pyrophosphate during the manufacture of liquid ammonium polyphosphate fertilizers, an important use of superphosphoric acid. As a consequence further refining is necessary to produce a satisfactory wet acid and numerous purification schemes have been suggested and put into practice over the years. Generally speaking, these fall into two categories, (1) extraction of the phosphoric acid with a solvent or (2) removal of dissolved metals from the crude aqueous wet acid.

In the extraction procedure (1) the wet acid is treated with an organic liquid such as an alkyl phosphate, aryl phosphate, alkyl aryl phosphate, ether, amine, alcohol or the like, and the resulting extract shaken with water which removes the phosphoric acid from the solvent. The aqueous solution of phosphoric acid is then concentrated to the desired strength.

Procedures falling in the second category involve treating the crude wet acid directly for the purpose of removing the metal contaminants. In this approach, various fiocculating or precipitating agents are employed capable of yielding insoluble compounds of the metal impurities and these are then removed usually by filtration and the resulting demetalized acid then concentrated to the desired strength.

An illustration of type 2 purification, to which the present invention pertains, is disclosed in US. 3,379,501 to Treitler et al. and involves adjusting the calcium ion level of 10-30% P 0 wet acid between 0.25 and 3% (as CaO) followed by the addition of hydrofluoric acid which reacts with the impurities and precipitates out the metal insolubles.

When the wet process is carried out using phosphate ore from the North Carolina deposits with their abnormally high magnesium content, the resulting wet acid presents an especially severe purification problem.

It has now been discovered that magnesium impurities in wet process phosphoric acid having an MgO/P O weight ratio from about 0.007 to about 0.03 and having a P 0 assay from about 42% to about 60% can be substantially reduced by treating the acid with hexafluorosilicic acid or other soluble hexafiuorosilicates thereby forming a precipitate of hydrated magnesium hexafluoro silicate (MgSiF -6H O) which is then removed leaving the purified acid. The treatment is carried out at a temperature range from about 20 C. to about 30 C. while the amount of soluble fiuorosilicate added is such that the weight ratio of Si:P O in the reaction solution is at least 0.003:1 and the weight ratio of F:P O is at least 0.03:1. An important and unique advantage of the invention is its utilization of hexafluorosilicic acid which can be produced as a by-product of the wet process.

In accordance with the present invention, crude wet acid is mixed with hexafluorosilicic acid whereby magnesium hexafluorosilicate (MgSiF -6H O) precipitates out in the form of a slurry which is easily separated from the acid and the latter then concentrated in the known manner. One manner of carrying out the processs is shown in the fiow sheet diagram of the single figure drawing. As can be seen from the diagram, filtration is employed to remove the MgSiF '6H O which, after washing, is then transported to a waste station or employed as a source of fluoride recovery. Washing is done by hexafiuorosilicic acid or other soluble hexafluorosilicate since the MgSiF 61-1 0 is highly soluble in water but relatively insoluble in the above solutions. Such Washing reduces P 0 losses in the filter cake which contains the wet acid. The wash solution can be used for further Mg precipitation or can be recycled back to the wet filter acid (before evaporation) in the wet acid plant. The filtrate is concentrated to give superphosphoric acid. Since the solubility of MgSiF 6H O increases with temperature, it is preferred to carry out the reaction at the lowest practice temperature; namely ambient temperatures in the range of 20-30 C. and most often at about 25 C. At higher temperatures, MgSiF -6H O solubility increases, creating post-precipitation problems if the filtrate cools after solids separation. Lower temperatures may be employed although the slurry may become viscous andpreent filtration difiiculties.

The process of the invention performs satisfactorily with acids whose P 0 content has been adjusted from about 42% to about 60% although optimum magnesium removal is best elfectuated with merchant grade acid, that is acid having P content of 52-54%. At the lower range of P 0 post-precipitation tends to occur in the final filtrate; at the higher range, fluorine containing vapors tend to be formed.

In order to avoid excessive dilution of the wet acid feed and wash liquor, it is preferable to use the highest possible concentration of HgsiF The acid is readily available commercially in concentrations of 28-30% and should be used as such since excessive dilution may cause increased solubility of MgSiF -6H O and consequent incomplete removal of the magnesium contamination. Also, additional evaporation will be required because of the excess of dilution in fluorosilicic acid.

The amount of H SiF used will vary with the particular source of a set acid since it will have varying quantities of magnesium and other impurities such as iron and aluminum. The overall amount of H SiF is 4-6 lbs./lb. of MgO in the wet acid. As already pointed out, the North Carolina phosphate rock is relatively rich in magnesium content and as a consequence wet acid derived therefrom will exhibit a high concentration of magnesium values. As a general rule, a silicon concentration by weight of at least 0.003:1 expressed as Si:P O and a fluorine concentration by weight of at least 0.03:1 expressed as F :P O in the reaction solution is required to initiate precipitation. With an acid of 53% P 0 60% of the magnesium, 25 of the iron and of the aluminum is removed using a total H 'SiF on a 100% basis, of about 0.04 parts per part of 53,% P 0 acid (by weight). Removal of still more of the metallic constituents requires greater quantities of the H SiF Those skilled in the art will use that quantity of H SiF needed to remove the metallic impurities down to the desired percentage to meet their needs.

The reaction of H SiF with wet acid is very rapid and is about 90% complete in 5 minutes and equilibrium is reached after approximately minutes.

As above pointed out, H SiF is an article of commerce which can be purchased on the chemical market. However, it is readily produced by condensing hydrogen fluoride and silicon tctrafiuoride which are by-products of the wet process. One method of recovery of fluorine values from the wet process in the form of HgSlFs is known as the Parrish process and is described in US. Pat. No. 3,091,513. Another source of soluble (SiF is ammonium hexafluorosilicate.

The present invention thus provides a means for removing impurities from wet acid and at the same time utilizes as the purification agent, H SiF which can be produced as a by-product during the acidification of the phosphate ore.

Reference is now made to the non-limiting examples which were carried out using the following procedures.

In the examples all tests were made in a 600 ml. beaker. Moderate agitation was supplied with a motor driven Teflon paddle impeller. The duration of all reactions was minutes. Final slurry filtrations were made through a 7 cm., diameter Bilchner funnel with polypropylene filter cloth into a vacuum flask. A high vacuum mechanical pump was used for suction. After filtering the Biichner funnel was emptied of its contents. When washing, the H SiF was poured over the filter cake and vacuum filtered into the Biichner funnel. All streams were weighed and analyzed for MgO, P 0 Si and F. Selected filtrates were also analyzed for Fe and A1.

A nominal 30% S SiF solution analyzing 23.96% F and 5.20% Si, was used in all runs. The wet acid was commercial North Carolina acid, containing 53.14% P 0 When testing other concentrations of P 0 this acid was either concentrated by evaporation or diluted water. Analyses of five acid concentrations used in the examples are given in Table I below.

TABLE I Constituent (percent by wt.) A B C D E (1) Effect of Feed Acid P 0 Concentration This is provided by the examples summarized in Table II. Examples 1-7 represent runs in which optimum results were obtained between starting acid concentrations of 45-60% P 0 With a 40% P 0 acid, no precipitation occurred (Example 8). With a 68% P 0 acid, no precipitation occurred and fluoride containing gases were evolved caused by the diminished solubility of HzSiFa in concentrated H PO The evolved gases were probably HF and/or SiF (Example 9).

(2) Fe and Al Removal Analyses of the filtrates in Examples 1-5 were made for Fe and Al, shown in Table III. Fe and Al were removed to the extent of about 25% and 10% of the amounts present in the feed acid, respectively.

(3) Filtration Rate Time of filtration of the end slurry was determined. In Examples 1-7, the filtration rate was rapid and was equivalent to about 150 gal. of slurry/hour-sq. ft. of filter area.

EXAMPLE 10 (4) Effect of Temperature EXAMPLE 11 (5) Rate of Reaction Reaction conditions were the same as in Example 5. Slurry samples, 20 ml. each, were taken every 5 minutes and filtered. The filtrates were analyzed with the following results:

Filtrate analyses Time of reaction (min.): (percent MgO) 5 0.33 10 0.32 15 0.30 20 0.30 25 0.28 30 0.28

The data shows that the reaction is very rapid with about of the reaction complete in 5 minutes. Equilibrium is reached in about 25 minutes.

TABLE II.-EFFECT OF FEED ACID CONCENTRATION (Examples 1-9) Filter cake wash Percent Acid used Wt. of Dis- Analyses (percent by wt.) 30% Gms. place- P205 MgO in Wt., Temp., HzSiFa, of 30% ment Constit- Wash loss in 72 Type grns. C. gms. IIzSiFfl washes uent Filtrate Residue liquor residue acid P O 48. 69 19 18 Example number. 7 5

1 A 300 2.429 43. 0 None htg g}? 2: $2 2.2. 0,37

F 1.51 27. 38 as g O 2 A 300 2030 37.6 None Si 16 480 2.5 0.44

F 1.45 25.73 {13%, 4g. (21g g4 11. g 2 0. 3 A 300 23-27 43.0 30 3 Si (L18 6374 L 0.74 0.34

F l. 43 27. 8 6. 85 e as g 0 1 4 A 300 23 27 43 0 6 Si 18 7'20 9 0.26 0.34

F 1.44 33. 46 19.95 at: 422. 2-22 g 0. 07 5 A 300 27 29 38.0 1.5 Si 0.18 5.93 L82 1.0 0.44

F 1.54 26.1 9 05 1164205 54.03 23.78 6 B 266 2431 43.0 None g; 2: as 0.36

F 1. 47 23.78 11342005 43. 11.4 g 4 so 7 C 354 43.0 None Si 0.46 4 37 1 2 0. 51

F 2.13 20.3 8 D 398 25 43. 0 No precipitate Obtained 9 E 300 50-60 43. 0 No precipitate formed-heavy foaming and fumes *Based' on P20 in wet acid feed. Calculated.

TABLE IIL-Fe AND A1 ANALYSES FOR FILTRATES IN EXAMPLES 1-5 0.270.28% in these solutions.

"Calculated values.

Nora-All values are percent by weight.

Pursuant to the requirements of the patent statutes, the principle of this invention has been explained and exemplified in a manner so that it can be readily practiced by those skilled in the art, such cxemplification including what is considered to represent the best embodiment of the invention. However, it should be clearly understood that, within the scope of the appended claims, the invention may be practiced by those skilled in the art, and having the benefit of this disclosure, otherwise than as specifically described and exemplified herein.

What is claimed is:

1. A method of removing magnesium impurities from wet process phosphoric acid having an MgO/P O weight ratio from about 0,007 to about 0,03 and having a F 0,;

assay of from about 42% to about comprising (1) treating the acid with a soluble hexafiuorosilicate (SiF in an amount such that'the weight ratio of Si:P O in the reaction solution is at least 0.003:1 and the weight ratio of F:P O is at least 0.03 :1 in the temperature range of 20-30 C. whereby magnesium hexafluorosilicate is precipitated and (2) separating the magnesium fluorosilicate from the purified acid.

2. The method according to claim 1 wherein the acid is merchant grade acid having a P 0 assay of 52-54%.

3. The method according to claim 1 wherein the soluble hexafluorosilicate is hexafluorosilicic acid.

4. The method according to claim 1 wherein the hexafluorosilicic acid is in the form of about 30% aqueous solution.

References Cited US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,819,810 Dated June 25 197 Inv nt r( David Goldstein It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 62, "practice" should read --pr*actical---.

Column 3, line 68, "S SiF should read --H SiF Column 6, Table II, "MgO in 72% Acid**" should read MgO in 72% Acid**--.

Signed and sealed this 24th day of June 1975.

(SEAL) Attest:

' C. l-IARSHALL DANN RUTH C. P'IASON Commissioner of Patents Attesting Officer and Trademarks F ORM PO-l 050 (0-69) USCOMM-DC 60376-P69 Q as. Govtmmzm rmutmo ornci "a o-au-au 

